The term “biomass” generally refers to the total amount of organisms that inhabit or exist in a given area. When such term is used for plants, in particular, the term refers to dry weight per unit area. A biomass unit is quantified in terms of a mass or an energy amount. In the case of plant biomass, the term “standing crop” is occasionally used to represent “biomass.” Since plant biomass is generated by fixing atmospheric carbon dioxide with the use of the solar energy, it can be regarded as so-called “carbon-neutral energy.” Accordingly, an increase of plant biomass is effective for global environmental preservation, the prevention of global warming, and mitigation of greenhouse gas emissions. Thus, technologies for increasing the production of plant biomass have been industrially significant.
Plants are cultivated for the purpose of using some tissues thereof (e.g., seeds, roots, leaves, or stems) or for the purpose of producing various materials, such as a fat and oil. Examples of fat and oil produced from plants that have been heretofore known include soybean oil, sesame oil, olive oil, coconut oil, rice oil, cottonseed oil, sunflower oil, corn oil, safflower oil, and rapeseed oil. Such fat and oil are extensively used for household and industrial applications. Also, a fat and oil produced from plants is used as biodiesel fuels, and the applicability thereof is increasing for alternative energy to petroleum.
Under such circumstances, it is necessary for the industrial success of the production of the fat and oil using plants that the productivity per unit of cultivation area be improved. If the number of cultivated plants is assumed to be constant per unit of cultivation area, an improvement in the amount of fat and oil production per plant is found to be necessary. When fat and oil are extracted from seeds obtained from plants, an improvement in the amount of fat and oil production per plant can be achieved via techniques of, for example, improving the seed yield per plant or increasing the fat and oil content in seeds.
Techniques for increasing the amount of fat and oil production from plant seeds are roughly classified into techniques based on an improvement in cultivation methods and techniques based on the development of plant varieties that can increase the amount of fat and oil production. Techniques based on the development of plant varieties are roughly classified as conventional breeding techniques such as crossing and molecular breeding techniques via genetic recombination. As techniques for increasing the amount of fat and oil production via genetic recombination, A) a method of modifying synthetic pathways for triacylglycerol (TAG) of seeds, which is a main component of plant fat and oil, and B) a method of modifying regulatory genes that regulate plant morphogenesis or metabolism are known.
In the method A) above, the amount of TAGs synthesized from sugars produced via photosynthesis can be increased by (1) enhancing synthesis activities of a fatty acids (i.e., TAG components) or a glycerol from sugars or (2) reinforcing the reaction of synthesizing TAGs from glycerol and fatty acids. In this regard, the following techniques have been reported as techniques using genetically engineering techniques. An example of (1) is a technique in which cytosolic Acetγ1-coenzyme A carboxylase (ACCase) of Arabidopsis thaliana is overexpressed in plastids of Brassica rapa L. ver. Nippo-oleifera and the fat and oil content in seeds is improved by 5% (Plant Physiology, 1997, Vol. 113, pp. 75-81).
An example of (2) is a technique of increasing the fat and oil production via overexpression of diacylglycerol acyltransferase (DGAT) that transfers an acyl group to the sn-3 position of diacylglycerol (Plant Physiology, 2001, Vol. 126, pp. 861-874). It is reported that the fat and oil content and the seed weight are increased as the DGAT expression level increases, and the number of seeds per plant may be occasionally increased according to the method of Plant Physiology, 2001, Vol. 126, pp. 861-874. The fat and oil content in Arabidopsis thaliana seeds was increased by 46% and the fat and oil amount per plant was increased by a maximum of about 125% by such technique.
As the method of B), expression of transcriptional factor genes associated with regulation of biosynthetic enzyme genes expression may be regulated. An example thereof is WO 01/35727. WO 01/35727 employs a technique in which recombinant plants are prepared via exhaustive overexpression or knocking out of transcriptional factors and genes that enhance the fat and oil content in seeds are then selected. WO 01/35727 discloses that overexpression of ERF subfamily B-4 transcriptional factor genes results in a 23% increase in the fat and oil content in seeds. WO 01/35727, however, does not disclose an increase or decrease in fat and oil content per plant. Also, Plant J., 2004, 40, 575-585 discloses the overexpression of WRINKLED1, which is a transcriptional factor having the AP2/EREB domain, improves the fat and oil content in seeds.
Although molecular breeding techniques as described above intended for the improvement of various traits have been developed, techniques for improving the yield involving increasing the weight of plant, increasing a given tissue, or improving the productivity of target substances have not yet been put to practical use.
Further, targets of techniques for increasing the production of target substances (fat and oil, in particular) via genetic recombination are dicotyledonous plants such as Arabidopsis thaliana and Brassica rapa L. ver. Nippo-oleifera. Techniques targeting monocotyledonous plants, such as rice and maize, are not yet known.
This is considered to be due to the following reasons. That is, truly excellent genes have not yet been discovered and new recombinant varieties that are found effective at the test phase cannot exhibit effects as expected during the practical phase under a variety of natural environments. In order to overcome such problems, the discovery of dramatically effective new genes and the development of genes exhibiting effects under practical environments, even if the effectiveness thereof is equivalent to that of existing genes, are necessary.